Abstract: Alloy particles were detected by XRD in bimetallic catalysts, made from physical mixtures of powders comprising distinct metal-support combinations (e.g., Pd on alumina and Rh on ceria-zirconia), following high-temperature redox aging. The morphology of the catalysts was examined by TEM, and the compositional structure of some of the alloy particles was determined. Two different effects of the redistribution of metals on oxygen-storage capacity were identified, one due to Pd enrichment of the surface of Pd-Rh alloy particles and the other due to loss of contact between metal and ceria-zirconia.